How to implement the Observer Design Pattern in a pure functional way?

Let's say I want to implement an event bus using a OO programming language. I could do this (pseudocode):

class EventBus

    listeners = []

    public register(listener):
        listeners.add(listener)

    public unregister(listener):
        listeners.remove(listener)

    public fireEvent(event):
        for (listener in listeners):
            listener.on(event)

This is actually the the observer pattern, but used for event-driven control flow of an application.

How would you implement this pattern using a functional programming language (such as one of the lisp flavors)?

I ask this because if one doesn't use objects, one would still need some kind of state to maintain a collection of all the listeners. More over, since the listeners collection changes over time, it would not be possible to create a pure functional solution, right?